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A Harvard study found that babies and toddlers who sleep fewer than 12 hours daily are at greater risk for being overweight in preschool, startling evidence that the link between sleep and obesity may affect even very young children.

TV viewing heightened the effect. The children who slept the least and watched the most television had the greatest chance of becoming obese.

“The two (behaviors) are acting independently. In combination, they are particularly risky,” said the study’s lead author, Dr. Elsie Taveras of Harvard Medical School.

The findings, published in April’s Archives of Pediatrics & Adolescent Medicine, are based on mothers’ reports of their babies’ sleep habits and TV viewing, and direct measures of the children’s height, weight and skinfold thickness.

Starting when the babies were 6 months old, mothers were asked how long their children napped during the day and how long they slept at night. Moms were asked again when the children were 1 and 2 years old. They were asked about TV time when the children reached age 2.

The researchers combined the sleep answers to find an average pattern for each child during the first two years of life. They found 586 of the children slept an average of 12 or more hours a day and 329 of the children slept less than that.

The short sleepers fared worse. Twelve percent of them became obese 3-year-olds. Adding TV to the picture, 17 percent of those who slept less than 12 hours a day and watched two or more hours of television a day were obese by the time they were 3.

Obesity was defined as having a body mass index in the 95th percentile or above. BMI is a measure that combines height and weight. A 3-year-old who is 3 feet, 3 inches tall and 40 pounds would be considered obese.

Double the risk for obesity
The researchers took into account other risk factors for obesity, including TV viewing, and still found the children who slept fewer than 12 hours a day had a doubled risk of being obese at age 3 than the other children.

Sleep’s impact on appetite hormones may explain the effect, Taveras said. In prior studies, sleep-deprived adults produced more ghrelin, a hormone that promotes hunger, and less leptin, a hormone that signals fullness.

TV viewing is thought to increase the risk of obesity both because it takes time away from calorie-burning play and because of food ads for snacks and fast food.

The families in the new study lived in Massachusetts and had relatively high incomes and education levels, making it difficult to apply the findings to everyone, Taveras acknowledged. Sleep researchers who read the study said it adds to growing evidence of the link between poor sleep and obesity. A study published last year found that every additional hour per night a third-grader spends sleeping reduces the child’s chances of being obese in sixth grade by 40 percent.

“The main message for parents is that there has to be regularity in sleep in children. It’s very important to maintain a schedule,” said Dr. Michelle Cao of Stanford University’s sleep disorders clinic. She wasn’t involved in the study but co-wrote an accompanying editorial in the journal.

Taveras recommended practices that teach infants to fall asleep on their own, putting them to bed when they’re drowsy but not fully asleep.

Pat Prinz of the University of Washington, who wasn’t involved in the study, said parents who rely on day care should make sure their toddlers have plenty of time to run, jump and play.

“The more active they are in the day, the better they’ll sleep at night,” Prinz said. But she cautioned that genetics may play a role in sleep and a person’s genetic makeup may limit how much sleep duration can be improved.